(1) Field of the Invention
This invention relates to a process for purifying a gas containing a sulfur compound and particularly to a desulfurizing agent suitable for removing hydrogen sulfide at a high temperature, and a process for treating a hydrogen sulfide-containing gas, a coal gasification system and a power generation system based on the coal gasification, using the desulfurizing agent.
Coal is widely distributed in all the world and its deposits are larger than the petroleum deposits. Coal has been regarded as a promising substitute energy for petroleum. However, much sulfur is contained in coal as a solid and its direct use as a fuel source is a problem because SO.sub.2 is generated by its combustion to pollute the environmental atmosphere. Recently, a system has been developed for gasifying coal in an oxygen-deficient state at a high temperature under a high pressure, removing useless substances such as sulfur compounds from the generated hydrogen and carbon monoxide gases and utilizing the produced gases as a clean energy source has been under development. Processes for removing the useless substances, that is, the sulfides generated from the gasification reaction, are classified into two process types, that is, wet process type and dry process type. The wet process type has been already established and has been practically utilized in various gasification furnaces or in the field of the chemical industry.
According to the wet process type, sulfides, particularly hydrogen sulfide, are removed by chemical absorption and it is necessary to cool the raw gas generated from the gasification reaction to allow an absorbing solution to absorb hydrogen sulfide. Energy losses due to the cooling, regeneration of absorbing solution, treatment of waste water, etc. have been problems.
In the case of the dry process type, on the other hand, hydrogen sulfide is removed from the generated gas at a high temperature without an absorbing solution, and thus there are no such problems as the energy loss due to the cooling and treatment of waste water due to the use of an absorbing solution. The dry process type has been thus regarded as a novel desulfurization process type.
The dry desulfurization process disclosed in Japanese Patent Publication No. 59-32169 uses a desulfurizing agent comprising titanium oxide and at least one of iron oxide and copper oxide. Though the disclosed desulfurizing agent has a good desulfurization efficiency, the oxygen in the iron oxide and the copper oxide as components of the adsorbing agent react with sulfur at an adsorbing temperature of more than 300.degree. C. to form sulfur dioxide. That is, there is a disadvantage of generation of sulfur dioxide during the desulfurization reaction.
Even in the treatment of a hydrogen sulfide-containing gas by a desulfurizing agent comprising titanium oxide and molybdenum oxide, disclosed in Japanese Patent Publication No. 59-19728, the oxygen in the molybdenum oxide reacts with sulfur, if the reaction temperature exceeds 300.degree. C., and the desulfurization efficiency is lowered thereby.
These desulfurizing agents have been developed for the treatment of Claus tail gas and thus the generation of sulfur dioxide during the adsorption of hydrogen sulfide has not been regarded as a problem. However, in the high temperature purification of a gas produced by coal gasification, it is not preferable that sulfur dioxide is generated during the adsorption of hydrogen sulfide. Furthermore, a temperature of more than 300.degree. C., at which no steam is condensed under pressure, is required during the desulfurization.
The process for treating a hydrogen sulfide-containing gas, disclosed in Japanese Patent Application Kokai (Laid-open) No. 59-223792, is directed to a gas produced by coal gasification and it is disclosed that an absorbing agent comprising titanium oxide thermally stabilized with tungsten oxide as a carrier, and at least one oxide of the iron group metals such as iron, nickel, cobalt, etc. and molybdenum, supported on the carrier, is useful, and that, though the hydrogen sulfide absorption rate at a high temperature is substantially equal to that of an absorbing agent containing no tungsten oxide, less deterioration of mechanical strength can be attained. When iron oxide is supported on the carrier and when the regeneration temperature is less than 538.degree. C., FeSO.sub.4 is formed and generates sulfur dioxide (SO.sub.2) during the desulfurization according to the following reaction: EQU FeSO.sub.4 +4/3H.sub.2 S.revreaction.FeS+4/3H.sub.2 O+4/3SO.sub.2
When the absorbing agent containing sulfur as adsorbed is regenerated, the adsorbed sulfur is released as a regenerated SO.sub.2 gas by an oxidizing gas such as oxygen, etc. and the regenerated gas is again subjected to reduction through contact with carbon, hydrogen, etc. in a treating unit such as a reduction column, etc. and is discharged as elemental sulfur through Claus reaction, etc. That is, a reduction column and a reducing agent such as carbon, hydrogen, etc. are required. That is, the regenerated gas is converted to sulfur or hydrogen sulfide through contact with carbon or hydrogen in a column called "reduction column" according to the following reactions (1) and (2): EQU SO.sub.2 +C=CO.sub.2 +S (1) EQU SO.sub.2 +3H.sub.2 =2H.sub.2 O+H.sub.2 S (2)
These reduction reactions require a temperature of more than 700.degree. C. and a relatively long reaction time. After the conversion of SO.sub.2 to S and H.sub.2 S by the reduction reactions, these gases are cooled to remove S therefrom, and the unreacted SO.sub.2 and H.sub.2 S, etc. are subjected to the following reaction (3) in a high temperature Claus reactor and a low temperature Claus reactor to remove S therefrom: ##EQU1##
The effluent gas from the low temperature Claus reactor is subjected to further treatment. The fixed bed process can attain an elaborate desulfurization, but requires a reduction column, a high temperature Claus reactor, a low temperature Claus reactor, a unit for treating the effluent gas, etc. to treat the regenerated gas. Thus, there are still operating problems.
The process for producing a gas having a low sulfur content produced by coal gasification, disclosed in Japanese Patent Application Kokai (Laid-open) No. 60-71690, is one of the dry process types, where a desulfurizing agent comprising iron as the main component and having particle sizes of less than 10 .mu.m is contacted with a raw gas in a temperature zone of 400.degree. to 800.degree. C. to fix the sulfides in the raw gas to the desulfurizing agent (reactive adsorption) to separate the sulfides from the raw gas. The process is more useful than the wet processes, but still has such problems that (1) the used desulfurizing agent is not made harmless and (2) the sulfur concentration in the purified gas is higher and its direct application range as a purified gas is limited.
The coal gasification process disclosed in Japanese Patent Application Kokai (Laid-open) No. 60-104188 proposes to solve the problem of said Japanese Patent Application Kokai (Laid-open) No. 60-71690, that is, the treatment of used desulfurizing agent, by charging the used waste desulfurizing agent into the high temperature zone in the gasification furnace, thereby saving the additional treatment of the used desulfurizing agent and also making the discharge of slag from the gasification furnace smooth. However, according to the proposed process, most of the desulfurizing agent is regenerated and recyclically used, whereas only the waste desulfurizing agent that can no more perform recyclic use is disposed in the gasification furnace. Thus, the proposed process fails to entirely solve the problems of said Japanese Patent Application Kokai (Laid-open) No. 60-71690. Furthermore, there is still such a problem that it is hard to separate the repeatedly applicable desulfurizing agent from the waste desulfurizing agent in other reactors than a fluidized bed reactor.
Hydrodesulfurization catalysts for separating sulfur in hydrocarbons as hydrogen sulfide are disclosed in Japanese Patent Application Kokai (Laid-open) No. 59-61949 and U.S. Pat. Nos. 4,490,479 and 4,272,400, where no mention is made of a desulfurizing agent for adsorbing hydrogen sulfide as sulfur from a hydrogen sulfide-containing gas, thereby removing the hydrogen sulfide from the gas, as in the present invention.
U.S. Pat. No. 3,928,238 discloses an oxidation catalyst for oxidizing unburned CO in a flue gas to CO.sub.2, where no mention is made of a desulfurizing agent as in the present invention.
As described above, the prior art still has such problems that sulfur dioxide is generated during the desulfurization. and the regeneration of used desulfurizing agent is an oxidative regeneration with oxygen, necessitating a reduction step based on a reduction column.